<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>
<body>
    
</body>
<script>
    //稀疏数组
    var arr1 = [,1,2,,,,3];
    console.log(arr1.length);
    for(var i in arr1){
        console.log(arr1[i]);
    }
</script>
</html>